Heat treatment and concentration of oxide ores



United States Patent Ofice Patented June 20, 1961 2,989,394 HEAT TREATMENT AND CONCENTRATION OF OXIDE ORES Leslie J. Bechaud, Jr., Danbury, and Herman Hartjens, Ridgefield, Conn., assignors to Newmont Exploration Limited, Danbury, Cnn., a corporation of Delaware No Drawing. Filed Dec. 19, 1958, Ser. No. 781,442 Claims. (Cl. 75-2) This invention relates to the heat treatment and concentration of oxidized base metal ores, such as ores containing the oxides of copper, lead, and silver, and has for an object to render the copper, lead and silver values contained therein readily recoverable by means of the conventional techniques normally. practiced in sulfide flotation.

The treatment of low grade oxidized ores of the base metals, with which may be associated more or less of a precious metal content, has long presented the met-allurgist or mineral dressing engineer with some of his most perplexing problems. The low grade ores containing, say, less than about 5% copper, lead, or the like, cannot be profitably treated by direct smelting because of the low metal content, nor do these ores usually respond to the normal techniques of physical concentration except in isolated instances. With low grade oxidized ores of copper, for instance, it has usually been necessary to resort to direct leaching accompanied by recovery of copper from solution by means of electrolysis, cementation, or precipitation on sponge iron or the like. Such plants are expensive and are not profitable except on a scale of operation of many thousands of tons of ore per day.

It has been proposed to roast finely ground oxide copper ore in admixture with pyrite at a temperature of 400 to 600 C. in a neutral or reducing atmosphere to form a magnetic compound containing copper, iron and sulfur. This process has drawbacks in the need for very fine grinding, intimate mixing, and results in a serious dust problem.

This invention is based on our discovery that the oxidized copper, lead and silver ores which may also contain other valuable metals can be made amenable to sulfide froth flotation by heating the ores in admixture with a halide and a sulfide in an atmosphere containing water vapor.

We have now found that if the roasting operation is performed in the presence of a halide such as sodium chloride, it is no longer necessary to grind the ore to a fine degree of dissemination, prior to heat treatment, and have in fact obtained substantial sulfidization of the oxide minerals at particle sizes as coarse as mesh. This we attribute to the probability that the halide forms volatile chlorides with the oxidized minerals and that the volatile chlorides then react with the pyrite to form sulfidized compounds with iron and sulfur. The sulfidized compounds are easily concentrated by the usual procedures of sulfide flotation.

In accordance with the process of our invention, the oxide ore is heated, usually in the temperature range of 500 C. to 800 C. but below the melting point of the ore, with a sulfide such as pyrite and in the presence of a halide such as common salt and water vapor, for a period of time usually not exceeding two hours, whereby the valuable oxide mineral constituents are converted to the sulfide form. The quantity of sulfide added should be at least suflicient to satisfy the sulfur requirements to convert metal oxide to sulfide. The salt and water vapor requirements are of the order of 1% to 2%. Pyrite can be substituted by other sulfides such as pyrrhotite, marcasite, arsenopyrite and by the alkali metal sulfides. Other halides may be used such as potassium chloride,

calcium chloride or magnesium chloride although sodium chloride is preferred because of its availability and low cost.

The presence of a free sulfur vapor, derived for instance from the free atom of sulfur in pyrite, sometimes results in a condition whereby the sulfur vapor reacts with the volatile chloride of the oxide metal and deposits metal sulfide particles indiscriminately on the surfaces of gangue and ore minerals alike. In a preferred method of our invention, the free sulfur is first driven from the pyrite before mixing with the oxide ore, for instance by heating the pyrite under a neutral atmosphere a separate heat treating furnace. Pyrrhotite is preferred to pyrite since there is no free sulfur in pyrrhotite. Similarly, the deleterious effect of the free sulfur in pyrite can be avoided by admitting to the reactor furnace only sufiicient oxygen to combine with the free sulfur, but

no more.

The heat treatment can be carried out in any suitable apparatus such as a multiple hearth roaster, rotary kiln, or other device which provides a means for control 'of temperature and furnace atmosphere.

In certain cases, where a suificiency of suitable sulfides as defined in the foregoing occur naturally in the ore under treatment, that is, where the natural ore minerals occur as both oxides and sulfides, the addition of sulfides such as pyrite or pyhrrhotite can be greatly reduced or eliminated.

The following examples illustrate operations carried out according to the process of our invention:

1. Oxide copper ore from Arizona This ore contained 2.07% total copper of which 1.64% was oxide copper (malachite and crysocolla) and 0.43% was sulfide copper. The ore was crushed to 10 mesh and mixed with 4% wt. pyrite, 2% sodium chloride, and 1% water. It was then roasted at 800 C. for 1 hour under a neutral atmosphere. After roasting, the calcine contained but 0.24% oxide copper, thus showing a high degree of conversion to the sulfide form.

The roasted material was ground to 48 mesh and subjected to froth flotation at pH 9.9 with amyl xanthate and frother.

A concentrate was recovered containing 9 /z% copper and accounting for 73.0% of the total copper in the ore.

2. Oxide copper are from Arizona The same material as treated in Example 1 was mixed with 1.7% wt. of pyrrhotite, 0.5% sodium chloride, and 0.4% water. The mixture was heated for 1 hour at 700 C. in a non-oxidizing atmosphere.

Upon grinding and floating for sulfide recovery, a concentrate was recovered accounting for of the total copper in the ore. The concentrate contained Il /2% copper.

3. Oxide tailing from South West Africa This material contained copper and lead values essentially in the form of arsenates, phosphates, carbonates and sulfates. Total lead content was 2.96% of which 2.77% was oxide lead; total copper was 2.07% and oxide copper 1.66%.

The material was mixed with 4% pyrite, 0.8% sodium chloride, and 0.4% water vapor and roasted at 700 C. for 1 hour under a neutral atmosphere. The roasted material was floated at pH 10.7 with ethyl xanthate, Aerofloat 15 and frother.

Total lead recovery amounted to 93.2% and copper recovery 82.4%. The cleaned concentrate assayed 17.6% Pb and 11.0% Cu.

4. Oxide lead-silver ore from South America This material contained only a trace of sulfides in the naturally occurring state and was almost completely oxidized. Lead values occurred principally as carbonates and sulfates.

The ore was crushed to. 20 mesh and heated in a neutral atmosphere with 5% pyrite and 2% sodium chloride for 1 hour at 700 C. Water vapor during roasting was provided by the water of crystallization present in the ore.

The calcine produced contained 3.05% Pb with only 0.29% as oxide lead. Silver in the amount of 13.96 oz. per ton was also present. This was ground to 48 mesh and floated with potassium amyl xanthate, Cyanamid Reagent 404, frother and soda ash for alkalinity control at pH 8.6. An over-all recovery was obtained amounting to 81.4% of the lead and 72.4% of the silver. The cleaned concentrate assayed 17.4% Pb and 68.3 02. silver.

These examples on ores drawn from widely scattered portions of the earth illustrate the applicability of the invention to various types of oxide ores.

We claim:

1. The process for the conversion of oxide base metal ore to convert the base metal oxides to sulfide compounds in a form amenable to sulfide froth flotation concentration which comprises heating a mixture of the ore in a state of subdivision and a metal sulfide and a halide of a metal of the group consisting of sodium, potassium, calcium and magnesium in an atmosphere containing water vapor and otherwise substantially non-oxidizing with respect to sulfur and sulfide to a temperature above 500 C. but below the melting point of the ore to produce a reaction with the base metals, the sulfide and the halide and form sulfides of the base metals.

2. In the process of claim 1, treating an ore in which the base metals are copper, lead and silver.

3. In the process of claim 1, providing in the mixture crushed and ground ore,- and heating the mixture in the range of 500 to 800 C. for a period not exceeding two hours.

4. In the process of claim 1, providing in the mixture a metal sulfide of the group consisting of iron sulfide, pyrite, pyrrhotite, marcasite and arsenopyrite, the quantity of sulfide being at least sufficient to convert the metal oxide to sulfide, a halide, and water in the amounts varying from about 1% to 2% by weight.

5. In the recovery of valuable base metals including copper, lead and silver from oxide ores, the improvement which comprises heating a mixture of ore, sodium chloride and a metal sulfide ground to a subdivided state, in the presence of water vapor, in an atmosphere nonoxidizing to sulfur and the sulfide at a temperature of from 500 to 800 C. for a time up to two hours to convert the base metals to sulfides which can be concentrated by sulfide froth flotation.

References Cited in the file of this patent UNITED STATES PATENTS 1,679,337 Moulden et al. July 31, 1928 2,878,102 Sternfels Mar. 17, 1959 FOREIGN PATENTS 255,961 Great Britain Aug. 3, 1926 

1. THE PROCESS FOR THE CONVERSION OF OXIDE BASE METAL ORE TO CONVERT THE BASE METAL OXIDES TO SULFIDE COMPOUNDS IN A FORM AMENABLE TO SULFIDE FROTH FLOTATION CONCENTRATION WHICH COMPRISES HEATING A MIXTURE OF THE ORE IN A STATE OF SUBDIVISION AND A METAL SULFIDE AND A HALIDE OF A METAL OF THE GROUP CONSISTING OF SODIUM, POTASSIUM, CALCIUM AND MAGNESIUM IN AN ATMOSPHERE CONTAINING WATER VAPOR AND OTHERWISE SUBSTANTIALLY NON-OXIDIZING WITH RESPECT TO SULFUR AND SULFIDE TO A TEMPERATURE ABOVE 500*C. BUT BELOW THE MELTING POINT OF THE ORE TO PRODUCE A REACTION WITH THE BASE METALS, THE SULFIDE AND THE HALIDE AND FORM SULFIDES OF THE BASE METALS. 